Steam Link is the more interesting of the two, as the app will reportedly allow gamers to experience their Steam games directly on their mobile devices. Steam Link for Android will support the Steam Controller, MFI controllers, and more.
The Steam Link app will be available from 21 May and will stream games as long as your device is connected to your home network over a 5GHz Wi-Fi connection or wired.
Streaming games from Steam to mobile devices isn't exactly new, as it's already possible via third-party apps. This app will let users watch Steam-based movies and shows "directly via their Android and iOS devices over Wi-Fi or LTE". This may not sound overly impressive compared to many other streaming options on the market, but Steam does host a surprisingly large selection of video content. We're still waiting exact launch dates for both apps, so stay tuned for updates.
But assume it does work, you'll be able to play GTA V on your phone with no downscaling or compromise and that's ruddy exciting.
As for the Steam Video App, it will be arriving sometime later this summer. It can also be used both in offline and streaming modes.
Steam is making it easier to access your library of games even if you are away from your desktop. Also, the company hasn't said what other kinds of controllers will be available for Link players - touch control would be nearly required for phone streaming, I'd think.